1. Field of the Invention
This invention relates to an X-ray imaging system for diagnostic use, and in particular to an X-ray radiography system including X-ray image intensifier tube and a video camera for pickup of the output image of the image intensifier tube.
2. Description of the Prior Art
The combination of an X-ray image intensifier tube and a video camera is employed in various diagnostic systems such as for example, X-ray television systems and X-ray radiography system. In a digital radiography (DR) system, video signals, obtained by use of an X-ray image intensifier tube and a video camera, are converted into digital data which is provided to an image processor. According to the Digital Fluoroscopic Angiography (DFA) technique disclosed in U.S. Pat. No. 4,204,225, contrast images of vessels are produced by subtracting post-injection image data from pre-injection image data.
Many commercial digital radiography systems employ X-ray image intensifier tubes having an image input diameter varying between 229 to 406 mm. The output image diameter of these tube is from 20 to 35 mm. The ratio of the input image to the output image (inverse number of image reduction ratio) exceeds 9.
X-ray image intensifier, tubes for performing direct fluoroscopic observation are known. The output image diameter of this type of tube is 100 mm and the ratio of the input image diameter to the output image diameter is 5.7. Another tube of this type has an output image diameter of 205 mm with the same input diameter as the 100 mm tube.
It is clear from investigation that the output image size of the X-ray image intensifier tube of the prior art digital radiography systems determines a limit of the spatial resolution of the systems. However, the prior art direct observation-type X-ray image intensifier tubes cannot be employed in digital radiography systems. The image detection part of a digital radiography system is mounted to a table on which a patient is positioned. The table has tilt and rotation mechanisms for obtaining X-ray images of the patient at various positions. Further, the height of the table when the table is level is limited to enable easy access. Therefore, there are practical limits for the dimensions of the image detection part of a digital radiography system. The prior direct observation-type X-ray image intensifier tubes have in particular large depths. Further, the output image diameter is too large raising the optical lens system for focusing the output image on a video camera to be too large dimensionally. If an X-ray image intensifier tube from a direct observation type X-ray image intensifier is employed in a digital radiography system, the dimensions of the image detecting part, which include an X-ray image intensifier tube, an optical lens system and a video camera, exceed the practical dimensional limits.
Accordingly, an object of this invention is to provide a digital fluoroscopy system having an improved spatial resolution and dimensions of the image detection part within practical limits.
Another object of this invention is to provide a digital radiography system having high sensitivity.
The image detection part of the digital radiography system according to this invention includes an X-ray image intensifier tube having an input image diameter of 254 to 457 mm, an output image diameter of 50 to 90 mm, a ratio of the input image diameter to the output image diameter having a range of 4 to 8, a video camera picking up the output image of the X-ray image intensifier tube, and an optical lens system focusing the output image of the X-ray image intensifier tube on the video camera.
Furthermore in accordance with the invention, a mirror for changing the optical path of the image is inserted between lenses of the optical lens system and the depth of the image detector part is between 700 and 800 mm.